	Related Topics:
	FMEAs

Risk Priority Numbers (RPNs)

Xfmea supports both of the risk assessment methods that are commonly used in conjunction with FMEAs: Risk Priority Numbers (RPNs) and criticality analysis. This chapter describes Xfmea’s capabilities for RPNs and related metrics such as SxO, SOD and SD. The Criticality Analysis topic presents Xfmea’s support for criticality analysis patterned after the MIL-STD-1629A guidelines.

Depending on how you have configured the analysis project, Xfmea allows you to use predefined rating scales in order to:

Rate the severity of each effect.
Rate the likelihood of occurrence for each cause.
Rate the likelihood of detection for each cause.

For each rating scale, the higher value represents the greater seriousness or risk. For example, with 10-point scales, a severity rating of 10 might indicate that the consequences of failure are very serious, an occurrence rating of 10 might indicate that the failure is very likely to occur and a detection rating of 10 might indicate that the failure is very unlikely to be detected before it reaches the end user.

Based on the ratings that have been defined for the effects and causes in the FMEA, you can configure the analysis project to calculate and display any of the following metrics:

RPN: This is the product of severity x occurrence x detection.

SxO: This is the product of severity x occurrence.
SOD: This is not a calculated value. Rather, it is the result of displaying three values in the following format: [Severity][Occurrence][Detection]. For example, if the severity is 7, the occurrence is 5 and the detection is 6, then the resulting SOD will be 756. When the issues are sorted in descending order, they will be prioritized first by severity, then by occurrence and then by detection.
SD: This is similar to SOD except that it displays Severity and Detection only.

Finally, Xfmea’s Risk Ranking feature allows you to define a more complex logic for ranking issues based on rating scales and/or metrics. There are many different ways that this logic could be configured. For example, you might wish to configure the analysis project to label the issue "High Priority" if the RPN is => XXX AND the severity rating is => Y. Alternatively, you might wish to label the issue "Action Required" if the severity rating is => X, the occurrence rating is => Y and the detection rating is => Z. And so on.

This chapter describes how to: